Graffiti is the illegal practice of painting, drawing or writing on walls, trains or other surfaces that do not belong to oneself, usually in public places. Graffiti can range from crude, untidily scrawled slogans to elaborately painted murals. Most graffiti is considered to be vandalism, although some practitioners devote considerable effort and undeniable talent to their 'work' and claim that it should be judged as art.

A common form of graffiti is 'tagging', which involves writing one's name or nickname in highly stylised letters. Other written graffiti may contain political messages; political graffiti is especially common in Western European towns and cities. Graffiti can appeal to groups who see themselves as disenfranchised, as it enables them to have a 'voice' without representation in the media or in government.

Some see graffiti as a part of modern 'street culture' (often associated with skateboarding or hip-hop and related pastimes). However, graffiti appears to be as old as civilisation itself - it has been found in the ruins of the Roman city of Pompeii, for example.